Aromatic polyimides are widely used thermosetting polymers having properties especially suitable for high temperature, high performance applications such as those encountered in spacecraft and missile uses. They are employed as binders for composite molded components requiring high thermaloxidative resistance and as surface protection coatings.
Recently, various structural modifications in the aromatic polyimides have been attempted. These all have as their aim an optimization of and/or balance between the desired end use properties such as thermaloxidative stability, mechanical strength, etc. and ease of processing.
Much attention has been focused on aromatic polyimides having the following base structure: EQU &lt;N--Ar--R--Ar--N&gt;
Ar is generally a phenyl ring or a substituted phenyl ring. The R-bridging group can be chosen from a wide diversity of possibilities. Typical structural modifications attempted in the past include changes in the structure of Ar, R, as well as the two groups attached to the terminal N atoms.
In addition, some attention has been given to the orientation of attachment of the R group to the two aryl groups. For example, Bell et al, Journal of Polymer Science: Polymer Chemistry Edition, Volume 14, 2275-2292 (1976), discloses the variation in polymerizability and polymer properties with differing orientations, i.e., p, p'; m, m'; o, o'; p, m'; p, o'; etc. Of particular note was the finding that imide monomers having at least one o-orientation were significantly more difficult to polymerize due to inductive and steric effects. Such results were shown in two series of polyimides, i.e., those based upon the diaminodiphenylmethanes and the diaminobenzophenones.
One subclass of polyimides having the structure depicted above is the polybismaleimide system. The base structure of the monomer is: ##STR2## or, alternatively, such compounds wherein the bridging group is a single substituted or unsubstituted aromatic ring instead of --Ar--R--Ar--.
Polybismaleimide systems having the depicted structure are disclosed, e.g., in Stenzenberger, Journal of Applied Polymer Science: Applied Polymer Symposium, Vol. 31, 91-104 (1977) or Stenzenberger, "The Preparation and Properties of High Performance Polyimide Composites", NASA Conference of Polymeric Materials for Unusual Service Conditions, NASA-Ames Research Center, Nov. 29-30-Dec. 1, 1972 (4,4'-bismaleimidodiphenylmethanes; 4,4'-bismaleimidodiphenylethers); U.S. Pat. No. 4,116,937 to Jones et al (R=--S--, --CH.sub.2 --, --C.sub.2 H.sub.4 --, inter alia; Ar=phenyl); and U.S. Pat. No. 4,100,140 to Zahir et al (R=--S--, --CH.sub.2 --, --C(CH.sub.3).sub.2 --, inter alia; Ar=phenyl). None of these disclose o,o'-compounds, ostensibly in view of the prejudice mentioned by Bell et al, supra, that such compounds are most difficult to polymerize to high molecular weights and/or have insufficiently high properties.
All of the prior art polybismaleimides have a common significant disadvantage, i.e., they all begin to polymerize at a temperature which is at or just above the melting point of the monomer. As a result, it is not possible to obtain a homogeneous melt prior to polymerization. The problem is especially severe in fabrication of large compression molded articles on the order of several or more inches thick. When a mold containing the bismaleimide is heated to the melting temperature, the outer portions of the mold reach the set temperature before the center of the mold; therefore, the outer portion of the matrix cures before the inner portion begins to melt. Cracks, voids and other defects easily develop under these conditions when final mold pressures are applied.
State of the art methods of overcoming these difficulties are unsatisfactory. For example, Grundschober et al (U.S. Pat. No. 3,533,966) require the addition of an extra ingredient into the mold in order to increase the pot-life of the monomer, i.e., the time period in which the (heretofore polymerizing) monomer can be poured. Kwiatkowsky et al (Journal of Polymer Science: Polymer Chemistry Edition, Volume 13, 961-972 (1975)) have attempted to improve flow properties by incorporating a polysulfone structure in the bridge between the maleimido groups. Both of these solutions are inadequate for many applications.
Another problem complicating the design of new polybismaleimides composites is the carcinogenic nature of many aromatic diamines including the bismaleimides. This limits the variety of commercially feasible alternatives.